In certain aircraft, turbine engines are mounted to pylons depending from the aircraft wings. Generally, fore and aft mounts and related structures connect the engine to the pylon in a manner that accommodates a variety of engine mounting requirements. In one known type of forward engine mounting arrangement, a stub shaft extends forwardly from the wing pylon and is received in a bore in a bulkhead bearing plate carried at the upper front end of the engine. The engine is thereby firmly connected to the wing pylon.
While the aforementioned mounting structure may function satisfactorily to connect the engine to the wing pylon, it is less than desirable because the direct metal-to-metal interconnection permits engine noises and lower frequency vibrations to be transmitted into the interior of the aircraft via the pylon and related wing structure. Engine noise and lower frequency vibration transmission can be reduced by providing a soft elastomeric connection between the stub shaft and engine. A soft connection is undesirable, however, because it can create engine whirl and flutter vibration problems.
Fluid-filled mounts have been used in a variety of applications, including the mounting of engines to frames in land vehicles, such as automobiles. Such mounts generally include a pair of opposed variable volume fluid-filled chambers separated by an elongate inertia track passageway which provides a fluid mass of a desired configuration that enables the mount to be designed, or tuned, to provide desirable dynamic operating characteristics. There are essentially two kinds of fluid mounts: a so-called single pumper type and a so-called double pumper type.
In a single pumper fluid mount, one of the chambers has a high volume stiffness relative to the other. In such a mount, changes in the volume of the fluid due to various factors, including changes in ambient temperature, are accommodated by changes in the volume of the chamber having the lower volume stiffness. In a double pumper type of fluid mount, both opposed fluid-filled chambers have a relatively high volume stiffness (in excess of about 100 psi. per cubic inch) to provide certain desirable dynamic operating characteristics. While this type of fluid-filled mount can be made relatively stiff so that it could avoid the aforementioned whirl and flutter problems, and provide a fluid tuned mass absorber effect at operating frequency, it has certain disadvantages in that changes in the volume of the fluid in the chambers, such as may be caused by variations in ambient temperature, can cause significant changes in working fluid pressure, thereby adversely affecting the performance of the mount. Changes in fluid volume can also be caused by migration of the fluid into the elastomeric material forming the flexible walls of the chambers. When this occurs, the desired volume stiffness is lost, and mount performance is adversely affected.
Fluid-filled mounts for various applications have been sold by the assignee of the present application under the trademark FLUIDLASTIC. While the fluid-filled mounts have found increasing usage in a variety of applications such as the automotive engine mounting applications discussed above, and to some extent in helicopter pylon mounting applications, their use in aircraft engine mounting applications is limited by the need to accommodate the aforementioned changes in volume of working fluid in the opposed chambers in response to the wide range of variations im ambient temperature encountered by modern turbine powered aircraft. Heretofore, a satisfactory fluid-filled mount capable of meeting the aforementioned turbine engine mounting requirements has not been available.
An attempt has been made to provide a vibration isolator capable of accommodating changes in volume of a working fluid due to changes in ambient temperature. Such a structure is described in U.S. Pat. No. 3,107,752 issued to the assignee of the present application. U.S. Pat. No. 4,613,118 and German published application No. DT 906282 both disclose so-called single pumper fluid mounts wherein changes in fluid volume are accommodated by changes in the volume of the chamber having the lower stiffness. U.S. Pat. No. 4,384,700 discloses an hydraulic system for damping movement of a vehicle seat by incorporating within the system an air precharged accumulator chamber. U.S. Pat. No. 3,807,678, also owned by the assignee of the present application, discloses a fluid system having an accumulator chamber which is connected to a main fluid system via valving means.